Amplifier output stages are frequently subjected to safe operating area (SOA) violations while driving loads in many applications. During an SOA violation, the peak junction temperature of the driver device exceeds the absolute maximum junction temperature of the driver device and can cause device destruction.
A conventional method to protect against SOA violations is to use a temperature sensor near the output driver device. The temperature sensor senses an average temperature in the vicinity of the output driver but is not able to sense and report the peak temperature excursions.
FIG. 1 is a plot showing the difference between the temperatures 10 sensed by such known temperature sensors in the vicinity of the output driver, and the actual temperature 11 of the driver device, when the output driver is subjected to large pulse power dissipation that causes an SOA violation. The actual temperature (11) of the driver device is measured using a thermal IR camera. As shown in FIG. 1, the temperature 10 sensed by the temperature sensor generally differs by at least 50-80° C. from the actual peak junction temperature (11), depending upon power dissipated, e.g., as measured by thermal IR camera. If the absolute maximum junction temperature is 150° C., which is a typical figure for many output driver devices, an SOA violation is not detected by the temperature sensor as shown in FIG. 1, even though the actual junction temperature exceeds 150° C.
Another method for protecting against SOA violations involves the use of an embedded temperature sensor. However, there are at least two major disadvantages to the embedded temperature sensor approach. First, such methods are generally specific to high power bipolar processors. For a DMOS (Double-Diffused-Metal-Oxide Semiconductor) process, the embedded temperature sensor is very difficult to implement and is prone to false trip and latch up issues caused by triggering of parasitic junctions. Secondly, depending upon layout, the embedded temperature sensor may also suffer from significant inaccuracies in measured temperature, as compared to the actual peak junction temperature. FIG. 2 shows a thermal simulation using finite element analysis (FEA) at 18 W peak power dissipation for an exemplary output driver device with an embedded temperature sensor in the center of the device. As shown in FIG. 2, the temperature 21 sensed by the embedded temperature sensor is 113.4° C., while the maximum actual peak junction temperature 22 is 129.4° C. Thus, the temperature 21 sensed by the embedded temperature sensor is at least 16° C. lower than the actual peak junction temperature 22.
Overcurrent protection is another known technique in amplifiers for protecting the output devices against SOA violations. However, overcurrent protection or current limiting is generally not adequate to protect against SOA violations. In many applications using reactive loads, load current and output voltage can have an out-of-phase relationship or phase delay between them. Thus, an SOA violation can occur at much lower output current levels that are well below the overcurrent trip threshold if there is a higher voltage across the output driver device.
Thus, a need exists in the industry to address the aforementioned deficiencies and inadequacies.